This invention relates to marine instruments and in particular to speed sensors for providing information with respect to marine vehicles traveling through water.
Boucher et al. in U.S. Pat. No. 4,555,938 describes a transom mounted marine speed sensor of the paddlewheel type, in which the paddles or blades are asymmetric in shape and formed of magnetized amorphous magnetic material. As the marine vessel passes through water, the paddlewheel rotates about an axis which is transverse the direction of travel. A Hall-effect device adjacent the paddlewheel senses the change in the magnetic field emanating from the paddles and generates an electrical signal directly proportional to the rotational speed of the paddlewheel. Generally, the rotational speed of the paddlewheel is linearly related to the vessel speed, but this is not always the case; especially at the extremes of high and low speeds.
Maeder et al. in U.S. Pat. No. 3,457,782 discloses a "through-hull" type sensor which employs a waterwheel having a plurality of buckets, closed at their side, and recessed in the peripheral edge of the wheel. The slot within which the wheel rotates is open on the downstream side. According to Maeder et al., this provides a relief space which prevents a buildup of pressure on that side, which would vary the rotational characteristics of the wheel.
Boucher in U.S. Pat. No. 4,898,029 discloses a through-hull paddlewheel sensor mounted in a housing which projects below the boundary layer of water formed on the hull of a boat as the boat traverses the water. The leading edge, i.e., the surface upstream of the paddlewheel, and trailing edge are co-planar.